Internal combustion engines convert chemical energy in a fuel to mechanical energy. As part of the conversion, the fuel can be combusted, thus causing hot combustion products to expand within the engine. The expansion of the combustion products can be used to move mechanical components of the engine, such as pistons. Combustion reactions can have several products, or emissions, some of which can be undesirable. For example, when hydrocarbons are used as fuel, combustion products can include HC, CO, CO2 and NOx.
In an attempt to reduce emissions, efforts have been made to utilize substantially carbon-free hydrogen as a combustion fuel instead of hydrocarbons. When hydrogen (H2) is used as the fuel, there is not a substantial amount of HC, CO, or CO2 emissions, because the fuel does not include carbon that can be turned into HC, CO, or CO2. The carbon based emissions that can exist come from the small amount of oil consumed during engine operation.
The inventor herein has recognized that several issues are raised by using substantially carbon-free hydrogen as a combustion fuel. In particular, while hydrogen fueled engines have low levels of HC, CO, and CO2 emissions, thus far, hydrogen engines have proven to have significant levels of NOx emissions. If a hydrogen fueled engine is operated at stoichiometry (34.2:1) in a port fuel injected, naturally aspirated engine configuration, NOx engine emissions can range from about 8500 ppm to about 10000 ppm, which is about 2.5 to 3 times the amount of a gasoline engine. Under such conditions, the engine is also very susceptible to auto-ignition and the performance level will be reduced by about 14% compared to a comparable spark-ignited gasoline engine.
While the inventor herein has recognized that operating a substantially carbon-free engine in a lean condition can reduce NOx emissions, the inventor has also recognized that running such an engine lean can make it difficult to achieve comparable power output. In an attempt to compensate for decreased power output in internal combustion engines, attempts have been made to increase engine displacement. However, such an approach can increase the weight of the engine, which can adversely affect vehicle fuel efficiency.
At least some of the issues associated with using substantially carbon-free fuel may be addressed by combusting a substantially carbon-free fuel at an equivalence ratio less than 1, while boosting intake air pressure. In this way, it may be possible to limit HC, CO, CO2, and/or NOx emissions, while maintaining satisfactory power output and vehicle fuel efficiency.